The present invention is related to and is specifically directed to a process control in an apparatus and in a system for separating a liquid/solid slurry.
The present invention relates to equipment of the kind in which the solids are removed from the slurry and are concentrated in the separating equipment.
In equipment of this kind, an influent solids ladened slurry is pumped, or otherwise fed, to an inlet of the dewatering equipment. These solids are removed from the slurry in the dewatering equipment and are concentrated within the dewatering equipment so that a relatively solids-free effluent flows from the outlet of the dewatering equipment.
The present invention relates to and is used in both continuous flow and batch flow dewatering processes.
Thus, the present invention relates to continuous flow dewatering processes such as belt presses, centrifuges, vacuum filters, sand filters and other liquid/solids separation equipment.
The present invention also relates to batch flow or cycle liquid/sold separation processes such as those utilizing a filter press.
However, in most batch flow processes some control of the pumping is usually desirable, to maximize the build-up of the cake solids in the shortest period of time or to avoid choking of the filter cloth.
A programmed control of the pump pressure build-up to avoid choking of the filter cloth is disclosed in detail in co-pending U.S. application Ser. No. 836,506 filed Sept. 26, 1977, by Zuckerman et al., and entitled "Slurry Dewatering Apparatus and Control System" and assigned to the same assignee as the assignee of this present application.
The present invention also relates to dewatering processes involving different types of liquid/solid slurries or sludges, such as relatively incompressible sludges and relatively compressible sludges. The present invention also relates to slurries which may require the addition of conditioning agents to enhance the separation process (such as organic municipal sewage sludges) and to other types of slurries that generally do not require the addition of conditioning agents (such as inorganic coal slurries and paper mill primary sludges).
There is a need for process control in both continuous flow and in batch flow dewatering applications. In the prior art much of the operation of the dewatering equipment is based on pilot studies or preset operating parameter set from similar applications or fixed during start-up which do not take into account or provide for changing conditions occurring in the actual operation of dewatering processes.
As a result, the prior art separation methods and apparatus have presented a number of problems.
In some cases the prior art methods and apparatus have not produced a product to meet certain requirements, for example, a high enough percent of solids concentration to meet requirements for land fill or for incineration of the filtered solids without the need for supplemental fuel.
The prior art methods and apparatus in some cases have not produced maximum yields with a desired minimum requirement of power.
The prior art methods and apparatus in some cases also have not produced the throughput capacity required.
Some of the problems associated with prior art separation methods and apparatus can perhaps best be illustrated by considering a specific batch flow dewatering method and apparatus using a filter press.
The use of a filter press for dewatering slurries is a well known method.
A filter press dewatering system, generally speaking, consists of a filter press, a pumping system to feed slurries to the filter press and slurry conditioning system.
The major components of a filter press are the filter press frame, filter cloth, filter plates and the hydraulic closing system.
The filter plates are so designed as to form a series of cloth covered chambers. A solids ladened liquid or sludge is pumped to one side of the cloth, and relatively solid-free liquid is removed from the other side of the cloth.
The hydraulic cylinder and frame are so constructed as to keep the plates in a closed filter pack which allows the dewatering of the sludge to occur.
Dewatering in a filter press of this kind is accomplished by pumping the slurry under pressure to one side of the cloth and by removing filtrate from the other side of the cloth at essentially atmospheric pressure. The pressure differential causes the slurries to physically give up water.
Typical terminal sludge feed pressures are in a range between 7 to 15 atmospheres.
During the filtration cycle dewatered sludge builds up on the sludge side of the cloth until a cake is formed between adjacent filter plates.
Typical filter press pumping systems consist of piston pumps working in combination with an equalization tank, or high capacity centrifugal pumps working in combination with piston pumps, or piston or diaphragm pumps designed to pump at a maximum rate of flow that the filter press can receive.
The conventional prior art is to terminate the filter press filtration process under one of the following methods:
(a) A fixed, filtration cycle time based on a pre-set timer. Typical filtration times are 30 minutes to 3 hours. PA1 (b) Measure the flow of filtrate from the filter press and terminate the filter press operation when the filtrate flow falls below a pre-determined rate (for example, a certain number of gallons per minute per square foot of filtration area times the number of chambers). Typically, the minimum flitrate flow is calculated at 0.01 to 0.05 gpm/square foot of filtration area in the filter press. PA1 (1) Continuous monitoring of the actual increase of solids concentration within the dewatering unit, PA1 (2) Measuring the rate of change of solids concentration increase with respect to time within a filter press, PA1 (3) Controlled pumping of the influent slurry to a filter press to maintain a substantially constant flow rate in the effluent from a filter press, and PA1 (4) Control of the amounts of conditioning agents added to the influent slurry in response to one or more other conditions of the separation process and equipment.
The objective of filtration is to produce a slurry cake which is substantially dry.
In the prior art it has been possible to determine the dryness of the filter cake (at various intermediate points during the cycle) only after the filtration cycle has been completed. Thus, typically in the prior art, a pilot cycle was run and the resulting cake was then burned in an oven to determine the percent of solids as related to the percent of moisture. Then, knowing the final percent solids concentration of the cake and knowing the total overall time of the cycle, intermediate cake concentration values could be calculated for various times in the cycle on a time ratio or effluent volume flow ratio basis.
This method of calculating intermediate solids concentration values at various points of time during the cycle has a number of drawbacks.
It is (as pointed out above) an after the fact type of determination.
It also rested on a number of assumptions. For example, it assumed a constant sludge feed concentration, but in actual process applications the percent of solids in the influent slurry can and do vary substantially.
The prior art also assumed a specific gravity of 1.0 for the sludge, but in actual practice the specific gravity is always greater than 1.0 and varies.
Also, as a practical matter, the nature of the solids in the influent slurry can change in process; and this affects the dewaterability of the slurry. For example, an increase in the amount of incompressible sand or grit in municipal sewage, or a change in the nature of the sludge itself, such as the sludge becoming septic and thus more difficult to dewater, can have a substantial impact on the instantaneous increase in solids concentration in the filter press at any particular point in time or over any particular interval in time.
Thus, prior art method for terminating the filtration process in a filter press (on either a fixed, pre-determined time basis or on the measurement of flow of filtrate basis) could result in the actual cake solids concentration varying substantially from that produced during the pilot run or from the preset parameters from similar applications or fixed during start-up.